PROJECT SUMMARY Precision cancer medicine currently focuses on knowledge of the cancer mutation repertoire and the tailored application of drugs that target altered genes or pathways in individual patients, such as use of BRAF inhibitors in patients with BRAF mutant melanoma. Immune checkpoint inhibitors targeting the PD-1/PD-L1 pathway have shown dramatic and durable clinical responses in melanoma and others cancers, but robust predictive biomarkers are lacking and innate resistance is common. Thus, a critical need exists for more sophisticated ex vivo functional testing modalities that recapitulate human tumor biology to predict response to targeted and immune-based therapies to develop personalized treatment plans in real-time. We recently developed a novel approach for evaluating ex vivo response to ICB using murine- and patient-derived organotypic tumor spheroids (MDOTS/PDOTS) cultured in a 3-dimensional microfluidic system (Jenkins et al., Nature, under review). We demonstrated that spheroids isolated from fresh mouse and human tumor samples retain autologous lymphoid and myeloid cell populations, including antigen-experienced tumor infiltrating CD4 and CD8 T lymphocytes, and respond to ICB in short-term ex vivo culture. Improved understanding of the immune and tumor response to immune checkpoint inhibitors within the tumor microenvironment will facilitate efforts to identify predictive biomarkers/models for immune checkpoint blockade in real-time, as well as future efforts to screen for therapeutic combinations that enhance the response to immune checkpoint blockade, and may ultimately provide a platform for the `personalization' of immunotherapy. I am currently a Clinical Fellow in Medicine in the Dana-Farber/Harvard Cancer Center Hematology- Oncology Fellowship Program. Over 80% of my time is devoted to my ongoing research under the mentorship of Dr. David A. Barbie (DFCI, Broad Institute). The remainder of my time is devoted to clinical practice and clinical training, working primarily at the MGH Cancer Center in the Center for Melanoma. My goal is to successfully transition from senior fellow to research instructor, and ultimately to an independent investigator in a tenure-track position. I am applying for the K08 Mentored Clinical Scientist Research Career Development Award to provide the necessary training and funding support to achieve this goal. Under the mentorship of Drs. Barbie and Flaherty, and the guidance of my advisory committee (Drs. Wong, Janne, and Fisher), I will access to the necessary resources and training to develop a successful, independent research program over the funding period of the award.